While the simplest approach for observing an object is an observation by the naked eye, the naked eye is intended to detect the change in light intensity, and thus not suitable for the observation of an object which brings about no change in light intensity or an object which brings about a small change in light intensity. The same applies to common photographs and image sensors because the change in light intensity is detected for the photographs and image sensors. For example, biological cells, bacteria, gratings, waveguides, microscopic steps at the surfaces of objects, structure of the same color, etc. bring about no changes or only small changes in light intensity, and it has been thus difficult to observe the shapes thereof. In particular, biological cells have many clear and colorless intercellular components, and it has been thus extremely difficult to observe the shape and intercellular components of the biological cells.
Therefore, conventionally, biological cells are subjected to a pretreatment for dyeing the biological cells to visualize the shapes thereof or identify of each intercellular component depending on the degree of dyeing. While biological cells can be visualized by dyeing, the dyeing technique is not able to be used in some cases depending on targets. In addition, the pretreatment for dyeing requires time for immobilization, etc of biological cells, which is not a simple approach for observation. Furthermore, the dyeing may cause the biological cells to die or alter the biological cells, thereby resulting in the problems of failure to observe the biological cells under normal conditions and limitation to subsequent uses of the samples.
In the meanwhile, objects which bring about no changes in light intensity even change the phase of light in response to the difference in refractive index or the optical path difference in many cases. The biological cells, bacteria, gratings, waveguides, microscopic steps at the surfaces of objects, structure of the same color, etc. mentioned above are also included in phase objects which modulate the shape of light. In the case of such phase objects, it is possible to observe the phase objects by a phase-contrast microscope, a differential interference microscope, or the like converting relative phase information into intensity. In addition, as described in Non-Patent Document 1, techniques for measuring absolute phase information on phase objects have been also researched and developed. In Non-Patent Document 1, the closed-loop feedback technique is introduced into a Mach-Zehnder interferometer, and the entire surfaces of phase objects are scanned by the phase measurement system which is capable of measuring changes in phase in microscopic regions of clear and colorless phase objects with a high degree of accuracy, thereby measuring absolute phase information on the clear and colorless phase objects.    Non-Patent Document 1: Mai HANESAKA, Eriko WATANABE, Jun MIZUNO, and Kashiko KODATE, “2-D Phase Measurement System for Microscopic Objects using Phase Locking Technique”, Optics & Photonics Japan 2007, Nov. 26, 2007, p.272-273